Method for manufacturing fluorescent x-ray screens



SCREENS A. NAWIJN A ril 16, 1968 METHOD FOR MANUFACTURING FLUORESCENTX-RAY Filed Sept. 17, 1965 INVENTO'R United States Patent 4 Claims. (c1.117-7 This application is a continuation-in-part of the application Ser.No. 206,607, filed July 2, 1962, by the present applicant jointly withMartinus Pieter Visser, entitled, Fluorescent Image Screen and Methodfor Manufacturing Such a Screen.

The invention relates to the manufacture of fluorescent screens and,more specifically, to a method of manufacturing flat X-ray fluorescentscreens for use in radiofluorographic cameras or in X-ray imageintensifier devices of the type comprising an optical system forprojecting the X-ray image formed on a fluorescent screen onto thephotocathode of an electronic image intensifier tube.

The optical system used in such X-ray cameras and devices are generallyof externally high luminosity in order to reduce as much as possible theX-ray dose required to obtain a sufficient exposure of the photographicfilm or a sufficient brightness of the fluoroscopic image. As iswell-known, the depth of field of optical objectives of high relativeaperture is very small. It is therefore essential that the X-ray screenconforms exactly to the object plane on which the objective is focusedand that the flatness of the screen be as good as possible. Evendeviations from these conditions in the order of fractions of amillimeter may result in poor resolution of the phograph or thephotocathode image.

In the application Ser. No. 206,607 mentioned above it has been proposedto meet the condition of screen flatness by providing a screenconsisting of a soft elastic X-ray pervious membrane that carries alayer of fluorescent material and is held in flat stretched condition bya rigid supporting frame. The frame is preferably formed by a pair ofcoaxial rings between which the edge of the elastic membrane is clamped.In manufacturing such a screen the fluorescent layer can be applied tothe stretched membrane by the well-known technique of sedimentation.According to that method a column of liquid containing particles of thefluorescent material homogeneously dispersed therein is maintained overthe membrane so as to allow the particles to precipitate and form alayer on the membrane.

I have found that in the sedimentation process use can be made of theelastic properties of the membrane to produce in a simple manner ascreen having a characteristic that is often found very desirable inphotofluorographic cameras and optical X-ray image intensifiers. Awell-known phenomenon occurring in optical imagery systems is that ofvignetting, i.e. the reduction in elfective cross-section of obliquebeams compared to the axial beam which results in a considerably lowerillumination of image points near the border of the image. It has beenproposed heretofore, in the case of optical X-ray devices, butcompensate for the vignetting effect by the use of an X-ray screen thatexhibits a gradual increase of the thickness of the fluorescent layerfrom the center towards the edge of the screen. Since the efiiciency ofthe conversion of X-rays into light by a fluorescent layer is generallyimproved by increasing the layer thickness the edge portions of theX-ray image on such a screen will appear brighter than the centerportion and a more evenly illuminated picture may result in the finalstages of the apparatus. However, it has not been possible heretofore todevise a simple and workable method and means to produce the desiredgradual increase in fluorescent layer thickness.

-It is, accordingly, the principal object of the present invention toprovide a method for manufacturing a flat X-ray fluorescent screen foruse in optical X-ray devices, which has a gradually increasing thicknessof the fluorescent layer from the center towards the edges and isthereby capable of compensating for the vignetting causw by opticalsystems used in conjunction with such screens.

The invention broadly comprises the characteristic steps of stretchingan elastic membrane on a rigid supporting frame, positioning the frameso that the stretched membrane is in a horizontal plane, applyingpressure to the stretched membrane to thereby cause the same to bulgeupwards, applying to the membrane a column of liquid containingparticles of a fluorescent material for sedimentation onto the upperside of the membrane, and removing said pressure after sedimentation iscompleted.

The invention will be described in detail hereinafter, reference beinghad to the drawings in which:

FIG. 1 is a front view of the structure of a rigid supporting frame anda membrane stretched across the frame;

FIG. 2 is a cross-sectional view of a screen made in accordance with theinvention; and

FIG. 3 is a cross-sectional view of an arrangement preferably used inpracticing the method.

In FIGS. 1 and 2 a membrane 1 is shown consisting of an elastic materialwhich is as pervious to X-rays land/or light rays as possible. Materialsthat are sufficiently pervious to X rays or light rays (depending onWhether the phosphor layer will be applied to the side of the membranefacing the optical objective or On the side facing the X-ray tube,respectively) and have sufficient elasticity and tensile strength arecommercially available. Due to their elasticity and tensile strengthcertain plastic films are particularly suitable for this purpose. As anexample a polyethylene terephthalate film known under the trademarkMelinex may be mentioned which I have found to give good results in athickness between 0.05 and 0.5 mm. depending upon the diameter of thescreen. Melinex films of such thickness are perfectly transparent andcolorless and do not absorb X-rays to a noticeable degree.

The membrane 1 is stretched tightly in all directions on an annularsupporting frame 2. This can be done e.g. by pinching a piece of filmwhich is slightly larger than the screen to be produced at severalpoints of its circumference and stretching it as evenly as possible inall directions, after which the annular frame 2 is laid upon themembrane and attached thereto by the aid of a suitable glue. Theremaining margin of the membrane can be removed after that. A ring 3attached by means of some screws to the supporting frame 2 may be usedto prevent loosening of the glue joint. It will be evident that,alternatively, the membrane 1 could only have been pinched along itsedge between two rings of sufiicient rigidity instead of being glued tothe ring 2.

To the membrane 1 a fluorescent layer 4 of varying thickness is appliedby means of the arrangement shown in FIG. 3. The supporting frame 2 islaid with the membrane in horizontal position upon the edge of apressure basin 6, an O-ring being inserted between the two and heavyblade springs 8 being applied to form a fluid tight seal for the basin6. When a pressurized medium, which conveniently may be compressed air,is admitted under the membrane through a pipe 7 the membrane will bulgeupwards due to the excess pressure.

Upon the membrane a ring 9 is placed defining a space into which aliquid 10 is poured out in which the desired fluorescent material hasbeen dispersed homogeneously. The height of the liquid column graduallyincreases from the center towards the edge of the screen. By leaving theliquid undisturbed for some time the fluorescent material is allowed toprecipitate and a layer is formed on the membrane whose thickness variesproportionally to the local height of the liquid column. After thesedimentation is completed the excess pressure is removed so that themembrane returns to its flat condition, and the screen is completed inthe conventional manner by sucking off the excess liquid and drying thescreen.

It is to be understood that in the drawing various dimensions, such asthe thickness of the membrane and the fluorescent layer, the pitch ofthe membrane and the height of the liquid column, have been exaggeratedfor the sake of clarity. For instance, in a certain case, I use aMelinex film 0.075 mm. thick for a screen of approximately 12 inchdiameter, and I give the membrane a pitch of 8 mm. and apply a liquidcolumn of approximately mm. over the top of the membrane in order toobtain a variation of fluorescent layer thickness roughly in the ratioof 1:2 /2.

Although the kind of variation of fluorescent layer thickness that canbe achieved by curving the elastic membrane during sedimentation bymeans of fluid pressure appears very satisfactory to compensate for thevignetting of most optical systems, it will be evident that mechanicalmeans such as a suitable gauge can be used able to press the membrane toany desired shape in order to meet special requirements.

If the membrane is to be used on the X-ray side a reflective backing forthe fluorescent layer is preferably provided by applying a thin, elasticcoating of a suitable material, such as a plastic glue containingtitanium dioxyde, onto the membrane prior to sedimentation. In addition,such an intermediate coating may improve the aflixture of thefluorescent layer to the supporting structure.

What I claim is:

1. A method for manufacturing a fluorescent X-ray screen comprising thesteps of:

stretching an elastic membrane on a rigid supporting frame;

positioning said frame so that the stretched membrane is in a horizontalplane;

applying pressure to said stretched membrane to thereby cause the sameto bulge upwards;

applying to said membrane a column of liquid containing particles of afluorescent material for sedi mentation onto the upper side of saidmembrane;

and removing said pressure after sedimentation is completed to allow thecoated membrane to return to a fiat condition.

2. A method for manufacturing a fluorescent X-ray screen comprising thesteps of:

stretching an elastic membrane on a rigid supporting frame;

positioning said frame so that said membrane is in a horizontal plane;

applying a ditference in fluid pressure on both sides of said membraneto thereby cause said membrane to bulge upwards;

applying to said membrane a column of liquid containing particles of afluorescent material for sedimentation onto the upper side of saidmembrane; and removing said difference in fluid pressure aftersedimentation is completed to allow the coated membrane to return to afiat condition.

3. A method as claimed in claim 2 wherein said difference in fluidpressure is applied by admitting a pressurized fluid into a sealed spacethe top wall of which is formed by said membrane.

4. A method as claimed in claim 1 wherein prior to the sedimentation areflective intermediate coating is applied to said membrane. r

References Cited UNITED STATES PATENTS 2,836,101 5/1958 De Swast 350l93X 2,726,167 12/1955 Emmens ll733.5 2,695,964 11/1954 Schepker 2507l2,459,693 1/1949 Gordon 250-71

1. A METHOD FOR MANUFACTURING A FLUORESCENT X-RAY SCREEN COMPRISING THESTEPS OF: STRETCHING AN ELASTIC MEMBRANE ON A RIGID SUPPORTING FRAME;POSITIONING SAID FRAME SO THAT THE STRETCHED MEMBRANE IS IN A HORIZONTALPLANE; APPLYING PRESSURE TO SAID STRETCHED MEMBRANE TO THEREBY CAUSE THESAME TO BULGE UPWARDS; APPLYING TO SAID MEMBRANE A COLUMN OF LIQUIDCONTAINING PARTICLES OF A FLUOROSCENT MATERIAL FOR SEDIMENTATION ONTOTHE UPPER SIDE OF SAID MEMBRANE; AND REMOVING SAID PRESSURE AFTERSEDIMENTATION IS COMPLETED TO ALLOW THE COATED MEMBRANE TO RETURN TO AFLAT CONDITION.